Field of the Invention
The present invention relates to track-and-hold amplifiers and, more particularly, to track-and-hold amplifiers using an operational amplifier with switching between alternative feedback and input configurations to therefor change from one operating mode to another.
In signal processing, there is often a need to provide, for a selected duration of time, the immediately preceding value taken by a varying analog signal. A typical situation in which this occurs is in connection with providing digital representations of amplitude values of an analog voltage signal, i.e. analog-to-digital conversions.
For such conversions, a succession of amplitude values of a varying analog signal is selected for conversion to digital representations, each such amplitude value being acquired after a selected interval has elapsed since the preceding acquisition. Often, the best conversion result can be provided if the input signal to the converter maintains a substantially constant value at the input to the analog-to-digital converter during the conversion process. A well-known kind of circuit for providing, for a selected duration of time, a substantially constant value for each of the selected analog values is a track-and-hold amplifier.
The rate at which a track-and-hold amplifier can provide such substantially constant values, each based on the value of an analog signal applied to the amplifier input at a selected time, is determined by the amount of time the amplifier takes to settle into each of its two operating modes. One of these modes is the track mode in which the amplifier output signal is substantially a reproduced version of the input signal ("tracks" this signal). The other mode is the hold mode in which the amplifier output signal provides a substantially constant value representing the input signal at the time of switching into the hold mode. A conversion to a digital representation during a hold period of the track-and-hold amplifier cannot be permitted to begin until the transients, occurring as the result of switching into the hold mode, have expired and a substantially constant voltage has been reached at the amplifier output for the duration of that hold mode. Thereafter, the track-and-hold amplifer, after having been switched back into the track mode of operation, cannot be permitted to be switched again into the hold mode until the transients occurring from the switching into the track mode have expired so that the amplifier output again represents the analog voltage currently appearing at its input.
Thus, these two settling times, added to one another, represent the shortest time interval between conversions that can be permitted if such a track-and-hold amplifier is to provide reasonably accurate values for the subsequent conversion to digital equivalents. The reciprocal of this sum of settling times is the highest conversion frequency which can be accurately achieved. Similarly, in other uses, these settling times limit the rate that a track-and-hold amplifier can switch between operating modes.
A single signal at a single input amplifier using an operational differential amplifier, as opposed to a differential signal, will result in imbalances in the stored charge on parasitic capacitances occurring at this input which will lead to longer settling times. Thus, there is desired a track-and-hold amplifier configuration which will counteract such effects to reduce settling times.